An oil pump for an automatic transmission includes a drive gear driven by a pump drive hub which is connected to a pump impeller of a torque converter and which is rotatably supported on a pump body through a bush, and a driven gear which is disposed so as to mesh with the drive gear and whose axial center is eccentric with respect to the axial center of the drive gear. These gears are disposed between a pump body fixed to a case of the transmission and a side surface of a pump cover fixed to the pump body. These gears are rotated in a space (pump chamber) defined between the pump cover and a pump plate fixed to the pump body by the pump drive hub and supply low-pressure oil (for example, −0.1 MPa) from a suction port to various parts as high pressure oil (for example, 1.8 MPa).
On mating faces between the pump body and the pump plate and between the pump plate and the pump cover, many oil passages which communicate to a discharge port and which supply the oil from an oil pan to the suction port are formed. Many oil passages which communicate to clutches, brakes or various types of valyes are formed intricately on the pump body, the pump cover and the pump plate.
In the automatic transmission, it is necessary to control hydraulic pressure in the oil passages finely and exactly in order to reduce shift shock. It is desirable to increase the diameter of the oil passages communicated to the clutches, the brakes and the valyes and to shorten of the length thereof. However, it is not able to adopt this measure in restricted space. In some cases, fine and long oil passages are adopted and thereby the delay in response or hydraulic pressure vibration are caused.
In order to avoid the delay in response or the hydraulic pressure vibration, it is desired that the oil passage from a control valye to the clutches, the brakes and the valyes is shortened and that the diameter of the oil passage in a valye body is increased. Therefore, a first regulator and a second regulator (pressure control mechanism) are provided on the oil pump and the diameter of the oil passage in a valye body is increased. Further, the control valye is disposed at a position where the regulator is disposed and the oil passages from the clutches, the brakes and the valyes to the control valye is shortened.
When the oil pump is driven at high speed, excess oil by the regulators is returned to an oil port formed on the oil pump and communicated to the oil pan. This structure is disclosed, for example, in “PRINCIPLES OF OPERATION” 1000/2000/2400 Series on highway transmission “P03065EN”, (USA), General Motors Corp., March 1999, foldout 15-16. Referring to FIG. 4, this structure is described as follows.
FIG. 4 shows a side face of a pump body 101 of prior oil pump. The pump body 101 includes an oil port 102 communicated to the oil pan, a first oil passage 103, an oil suction port 104, an oil discharge port 105 which supplies the pressurized oil in the oil chamber to various parts, a second oil passage 106 which is communicated to the first regulator and which returns the excess oil to the first oil passage 103 and a third oil passage 107 which is communicated to the second regulator and which returns the excess oil to the first oil passage 103. The numeral 108 shows the pump drive hub and the numeral 109 shows an oil recover passage.
The oil from the oil pan flows into the first oil passage 103 along a flow shown by A and the oil in the second oil passage 106 from the first regulator flows into the first oil passage 103 along a flow shown by B. Further, the oil in the third oil passage 107 from the second regulator flows into the first oil passage 103 along a flow shown by C. Namely, the excess oil from the regulators is not returned to the oil pan directly and is merged to the oil flow A.
In FIG. 4, the oil flow A heads toward the first oil passage 103. The oil flows B and C include downward components and ram the oil flow A from the side. Thereby, turbulent flow generates by three oil flows A, B and C and the limit of the cavitation is decreased. As a result, abnormal noise is generated at high speed rotation of the oil pump.